1. Field of the Invention
The present invention pertains to an image capture panel for capturing direct radiographic images. More particularly, the present invention pertains to a method and apparatus for eliminating residual electrical charges residing in the image capture panel prior to capture of a subsequent radiographic image.
2. Description of the Related Art
Traditional medical diagnostic processes record X-ray image patterns on silver halide films. These systems direct an initially uniform pattern of interrogating X-ray radiation through a patient to be studied, intercept the consequently imagewise modulated pattern of X-ray radiation with an X-ray radiation intensifying screen, record the intensified pattern in a silver halide film, and chemically transform this latent radiation pattern into a permanent and visible image, called a radiogram.
Radiograms have also been produced by using layers of radiation sensitive materials to directly capture radiographic images as imagewise modulated patterns of electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixelized area are quantized using a regularly arranged array of discrete solid state radiation sensors. U.S. Pat. No. 5,319,206, issued to Lee et al. on Jun. 7, 1994 and assigned to E. I. du Pont de Nemours and Company, describes a system employing a layer of photoconductive material to create an imagewise modulated areal distribution of electron-hole pairs which are subsequently converted to corresponding analog pixel (picture element) values by electrosensitive devices, such as thin-film transistors. U.S. Pat. No. 5,262,649 (Antonuk et al.) describes a system employing a layer of phosphor or scintillation material to create an imagewise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as amorphous silicon photodiodes. U.S. Pat. No. 5,254,480 (Tran) describes a system which combines a luminescent layer, to create a distribution of photons, with an adjacent layer of photoconductive material to create a corresponding image-wise modulated distribution of electrical charges which are subsequently converted to corresponding analog pixel values for the image by electrosensitive devices. These solid state systems have the advantage of being useful for repeated exposures to X-ray radiation without consumption and chemical processing of silver halide films.
In systems utilizing a photoconductive layer, before exposure to imagewise modulated X-ray radiation, the top areal surface of the photoconductive layer is uniformly biased relative to electrical charge read-out means by application of an appropriate electric field. During exposure to X-ray radiation, electron-hole pairs are generated in the photoconductive layer in response to the intensity of the imagewise modulated pattern of X-ray radiation, and these electron-hole pairs are separated by the applied biasing electric field. The electron-hole pairs move in opposite directions along the electric field lines toward opposing surfaces of the photoconductive layer. After the X-ray radiation exposure, a latent image in the form of an imagewise distribution of electrical charges of varying magnitude is captured within the photoconductive layer, representing a latent electrostatic radiogram. A plurality of charge capture elements and switching devices proximate the photoconductive layer are adapted to readout the imagewise distribution of electrical charges, thereby providing a pixelized radiogram.
A problem with such an electrical charge capture and readout scheme is that after exposure to X-ray radiation is stopped and the electronic charge distribution within the photoconductive layer is determined by readout, some of the electrical charges induced within the photoconductive layer may continue to reside as charges trapped not only within the photoconductive layer but also at planar interfaces between the surfaces of the photoconductive layer and adjacent layers. These residual electrical charges must be fully eliminated prior to the next X-ray exposure. Otherwise, a false image pattern related to the previous radiation pattern may be added to subsequent radiograms.
It is known to intentionally flash expose a photoconductive layer of an image capture element to a large dose of actinic radiation to eliminate residual electrical charges stored in the photoconductive layer by momentarily rendering the photoconductive layer substantially conductive, for example, as described in U.S. Pat. No. 5,166,524, issued to Lee et al. on Nov. 24, 1992 and assigned to E. I. du Pont de Nemours and Company. However, such an image capture element must be partially disassembled by physical separation of a conductive contacting layer, such as conductive foam or rubber, from an array of charge capturing microplates before the flash exposure is made. Also, a large neutralizing current may be locally created by the flash exposure and exceed the current capacities of nearby readout components. Residual charges have also been minimized by application of a reversed and decreasing electric field, for example, as described in U.S. Pat. No. 5,319,206. However, this process involves multiple applications of a decreasing and reversed electric field to fully neutralize residual electrical charges remaining in the photoconductive layer.